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ITGlossary · Electrical units
Power Factor
The ratio of real power (W) to apparent power (VA), from 0 to 1, showing how many watts a UPS can deliver per VA.
Technical level:Intermediate
In an AC circuit, not all the power that flows is turned into useful work. The power factor (PF) is the ratio of real power in W to apparent power in VA and takes values from 0 to 1.
For a UPS, the output power factor shows how many watts it can deliver per VA: a 1000 VA UPS with a PF of 0.9 delivers up to 900 W, whereas with a PF of 0.6 it delivers only 600 W. Modern computer power supplies with active PFC have a power factor close to 1, so the watt limit is usually reached first.
A common mistake is choosing a UPS on its VA rating alone. Always check the rated power in W too and compare it with your equipment's consumption in W.
Questions that mention it
All frequently asked questionsWhat is the power factor?
The power factor is the ratio of real power to apparent power and shows how efficiently electrical energy is used. Ideally it is close to 1. In UPS systems a high power factor is critical, because it makes better use of the UPS's available power and reduces the risk of overload.
Related articles
Related terms
- kVA (kilovolt-ampere)A unit of apparent power equal to 1,000 VA, used to rate UPS systems; real power in kW equals kVA × power factor.
- Load SheddingA method used to prioritize power delivery by disconnecting non-critical loads during outages.
- Three-phase PowerA supply with three AC phases shifted by 120° (400 V between phases), used for large loads, motors and UPS systems from about 10 kVA.
- Hardwired (Terminal Block) ConnectionPermanent connection of the UPS input or output by an electrician, with cables screwed into a terminal block instead of plugs and sockets, usually on higher-power UPS units.
- IEC C13/C14 ConnectorsStandardised pair of connectors rated up to 10 A under IEC 60320, found on UPS outputs and on the power supplies of computers, servers and monitors.
- IEC C19/C20 ConnectorsPair of connectors rated up to 16 A under IEC 60320, used on higher-power UPS units, rack PDUs and servers or switches with high-consumption power supplies.